An arrhythmia is an abnormal heart beat pattern. One example of arrhythmia is bradycardia wherein the heart beats at an abnormally slow rate or wherein significant pauses occur between consecutive beats. Other examples of arrhythmias include tachyarrhythmias wherein the heart beats at an abnormally fast rate. With atrial tachycardia, the atria of the heart beat abnormally fast. With ventricular tachycardia, the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, a tachycardia is typically not fatal. However, some types of tachycardia, particularly ventricular tachycardia, can trigger ventricular fibrillation wherein the heart beats chaotically such that there is little or no net flow of blood from the heart to the brain and other organs. Ventricular tachycardia, if not terminated, is fatal. Hence, it is highly desirable to prevent or terminate arrhythmias, particularly ventricular tachycardia.
One technique for preventing or terminating arrhythmias is to overdrive pace the heart wherein a implantable cardiac stimulation device, such as a pacemaker or implantable cardioverter defibrillator (ICD), applies electrical pacing pulses to the heart at a rate somewhat faster than the intrinsic heart rate of the patient. For bradycardia, the cardiac stimulation device may be programmed to artificially pace the heart at a rate of 60 to 80 pulses per minute (ppm) to thereby prevent the heart from beating too slow and to eliminate any long pauses between heartbeats. To prevent tachyarrhythmias from occurring, the cardiac stimulation device artificially paces the heart at a rate slightly faster than the intrinsic tachyarrhythmia heart rate of the patient. In other words, a slight artificial tachycardia is induced and maintained in an effort to prevent an actual tachycardia from arising. If an actual tachycardia occurs, such as a supraventricular tachycardia (SVT) wherein the heart may begin beating suddenly at 150 beats per minute (bpm) or more, the cardiac stimulation device senses tachycardia and immediately begins pacing at a rate of slightly faster than the tachycardia, then slowly decreases the pacing rate in an effort to slowly reduce the heart rate back to a normal resting rate thereby terminating the tachycardia.
In one exemplary technique, the stimulation device monitors the heart of the patient and, if two consecutive intrinsic heartbeats are detected, overdrive pacing is automatically triggered. The overdrive pacing rate is based on the heart rate detected at the time overdrive pacing is triggered and is typically 5 to 10 ppm higher than the intrinsic rate. The intrinsic heart rate may be determined, for example, by calculating the time interval between the two consecutive intrinsic beats. The stimulation device then overdrive paces the heart at the selected overdrive pacing rate for a dwell time consisting of a programmed number of overdrive events or cycles. Thereafter, the stimulation device slowly decreases the overdrive pacing rate by a rate decrement specified by a programmed recovery rate until additional intrinsic beats are detected, then the device repeats the process to determine a new overdrive pacing rate and pace accordingly. If the heart rate is increasing quickly, such as may occur with an episode of tachycardia, the stimulation device may still detect intrinsic beats even while overdrive pacing is being applied. If so, the stimulation device immediately determines a new higher overdrive pacing rate based on the selected response function and the new heart rate. Again, if intrinsic beats are still detected, the overdrive pacing rate is increased per the response function. In this manner, the overdrive pacing rate may quickly be increased to 150 ppm or more in response to a tachycardia such as SVT.
Ultimately, the overdrive rate will be increased to the point where it exceeds the intrinsic rate of the tachycardia and hence no intrinsic beats will be detected. The pacing rate is eventually decreased using rate recovery until two or more consecutive intrinsic beats are again detected and the pacing rate is increased again. Assuming that overdrive pacing has succeeded in terminating the tachycardia, rate recovery will ensure that the pacing rate decreases slowly back down to a normal rate of perhaps 60 to 80 bpm. If a base rate is programmed, such as 60 bpm, the heart will be paced at the base rate even if the recovery rate would otherwise cause the rate to decrease even further. Likewise, if an alternative base rate, such as the rest rate or circadian base rate, is programmed, the pacing rate will not fall below those rates either.
It is believed that overdrive pacing is effective for at least some patients for preventing or terminating the onset of an actual tachycardia for the following reasons. A normal, healthy heart beats only in response to electrical pulses generated from a portion of the heart referred to as the sinus node. The sinus node pulses are conducted to the various atria and ventricles of the heart via certain, normal conduction pathways. In some patients, however, additional portions of the heart also generate electrical pulses referred to as “ectopic” pulses. Each pulse, whether a sinus node pulse or an ectopic pulse has a refractory period subsequent thereto during which time the heart tissue is not responsive to any electrical pulses. A combination of sinus pulses and ectopic pulses can result in a dispersion of the refractory periods, which, in turn, can trigger a tachycardia. By overdrive pacing the heart at a uniform rate, the likelihood of the occurrence of ectopic pulses is reduced and the refractory periods within the heart tissue are rendered uniform and periodic. Thus, the dispersion of refractory periods is reduced and tachycardias triggered thereby are substantially avoided. If a tachycardia nevertheless occurs, overdrive pacing at a rate faster than a tachycardia helps to eliminate ectopic pulses and reduce refractory period dispersion, and thereby helps to terminate the tachycardia.
However, in order for this scheme to work, it must be assured that each overdrive pulse actually triggers an atrial contraction, i.e. that the overdrive pulses are captured by the atria. If overdrive pulses are not captured, i.e. a loss-of-capture (LOC) occurs, intrinsic pulses are typically generated within the heart. The intrinsic pulses may be ectopic pulses of the type triggering tachyarrhythmia. Moreover, even if a tachyarrhythmia does not occur, the presence of the intrinsic pulses may trigger unwanted increases in the overdrive rate resulting in a generally higher overdrive pacing rate than needed. A high overdrive pacing rate has certain disadvantages. For example, the high rate may be unpleasant to the patient, particularly if the artificially-induced heart rate is relatively high in comparison with the heart rate that would otherwise normally occur. A high overdrive rate may also cause possible damage to the heart or may possibly trigger more serious arrhythmias, such as a ventricular fibrillation. A high overdrive rate may be especially problematic in patients suffering from heart failure, particularly if the heart failure is due to an impaired diastolic function and may actually exacerbate heart failure in these patients. Also, a high overdrive rate may be a problem in patients with coronary artery disease because increasing the heart rate decreases diastolic time and decreases perfusion, thus intensifying ischemia.
In an attempt to avoid LOCs during overdrive pacing, conventional devices typically set the magnitude of the overdrive pulses to be quite high so as to assure that the overdrive pulses are captured. Typically the magnitude of each overdrive pulse is set to at least twice the expected capture threshold, i.e. twice the pulse magnitude actually expected to achieve capture. The need to apply overdrive pacing pulses with high pulse magnitude operates to deplete the power supply of the implantable cardiac stimulation device. Since overdrive pacing is preferably performed more or less continuously within many patients, the increased pulse magnitude can have a significant effect on battery longevity perhaps requiring frequent surgical replacement of the device.
Accordingly, it would be desirable to provide an overdrive pacing technique that permits a reduction in the average magnitude of overdrive pacing pulses while still achieving adequate capture to thereby reduce overall power consumption and enhance device longevity while also ensuring that adequate overdrive pacing therapy is delivered to reduce the risk of tachyarrhythmia. It is to this end that the invention is primarily directed.